Differential input probes are commonly used to acquire a nominally differential signal, such as on a high-speed serial data bus, from a device under test (DUT) using a single test and measurement instrument channel. TriMode™ probes have the additional capability of acquiring the common-mode signal, or either side of the differential pair signal as a single-ended signal.
An ideal differential signal includes two complementary signals sent on two separate wires. Any skew, or difference in delay, between the two sides of wiring in a DUT to the test and measurement instrument and/or within the test and measurement instrument itself causes mode conversion of the differential signal. Mode conversion is when a portion of the differential signal appears as the common-mode signal, or vice versa. Mode conversion due to skew grows progressively worse at higher frequencies. For instance, just one picosecond (ps) of skew at 25 GHz will lead to over 15% of the common-mode voltage appearing as a differential signal.
One approach to minimizing errors due to the skew-related mode conversion is discussed in co-pending U.S. application Ser. No. 14/745,757, titled TRI-MODE PROBE WITH AUTOMATIC SKEW ADJUSTMENT, filed Jun. 22, 2015, and incorporated herein by reference in its entirety.
The electronically variable delays in U.S. application Ser. No. 14/745,757 may be broad-band, DC-coupled, electronically adjustable analog delay lines implemented as lumped-element transmission lines using fixed inductors and varactors (voltage-variable capacitors). However, such type of electronically adjustable analog delay line causes the characteristic impedance to vary along with the delay, thus requiring the ratio of maximum-to-minimum delay to be no bigger than the allowed ratio of impedances to maintain satisfactory termination. This limited ratio generally leads to a nominal delay much longer than the needed delay range, which in turn leads to higher insertion loss than desired. Also, the varactors will respond to the signal voltage as well as the adjustable bias voltage, causing some signal non-linearity.
Another known approach is to build a segmented delay line with microelectromechanical systems (MEMs) switches configured to switch in or out different segment lengths to implement a stepped delay control. This approach avoids the change in characteristic impedance with delay, but requires a specialized manufacturing process and may suffer significant insertion loss due to contact resistance of the MEMs switches.
Yet another approach is the use of switched active delay elements (e.g., unity gain amplifiers that impart a relatively known delay to the signal). This approach is compatible with standard integrated chip (IC) processes, but degrades signal-to-noise ratio due to noise generation in the active stages, and may require more operational power than other approaches.
Embodiments of the disclosed technology address these and other limitations in the prior art.